Centrifugal pump

ABSTRACT

A two-stage centrifugal pump assembly adapted to produce high pressures from the second stage with a plurality of centrifugal sealing impellers associated with the second stage for the purpose of reducing the pressure drop across a mechanical seal associated with the second stage to minimize the second-stage sealing loss, there being provided a bypass from the low-pressure side of the first stage impeller to the sealing impellers of the second stage for delivering pumped fluid to the second-stage sealing impellers in a manner to prevent heating and boiling in the second-stage sealing impellers, there being also provided an alternate provision for supplying a noncorrosive compatible buffer fluid as the sealing fluid to the inlet of the sealing impellers in the second stage which also maintains the sealing impellers cool and at the same time isolates the impellers and the main impeller shaft from the corrosive fluid being pumped.

Stts 1 Beck, ,lr, et a1.

[54] CENTRllFlUGAL PUMP [73] Assignee: Sundstrand Corporation [22] Filed: Apr. 23, 1970 [21] Appl. No.: 31,197

152] U.S.Cl ..4l5/l09,415/112,415/176 [51 llnt. Cl ..F04d 29/08 [58] Field ofSearch AIS/108,109,110, 111,112, 415/113, 115, 116, 170 C, 175, 176; 277/3 [56] References Cited UNITED STATES PATENTS 790,683 5/1905 Harris ..415/11O 1,123,364 1/1915 Peterson ..415/100 2,663,262 12/1953 Malmuik et a1... ....1415/109 2,781,209 2/1957 Jacobs 1 ..415/111 3,213,798 10/1965 Carswell... ..415/l12 3,410,565 11/1968 Williams ..277/3 Mfl/NPUMP lNlET FLOW [ 1 Feb. 1, 1972 FOREIGN PATENTS OR APPLICATIONS Primary Examiner-Hcnry F. Raduazo At10rney-Hofgren, Wcgner, Allen, Stellman & McCord [57] ABSTRACT A two-stage centrifugal pump assembly adapted to produce high pressures from the second stage with a plurality of cen' trifugal sealing impellers associated with the second stage for the purpose of reducing the pressure drop across a mechanical seal associated with the second stage to minimize the secondstage sealing loss, there being provided a bypass from the lowpressure side of the first stage impeller to the sealing impellers of the second stage for delivering pumped fluid to the secondstage sealing impellers in a manner to prevent heating and boiling in the second-stage sealing impellers, there being also provided an alternate provision for supplying a noncorrosive compatible buffer fluid as the sealing fluid to the inlet of the sealing impellers in the second stage which also maintains the sealing impellers cool and at the same time isolates the impellers and the main impeller shaft from the corrosive fluid being pumped.

12 Claims, 2 Drawing Figures menu-Inna Hm asae'om SHEET 2 OF 2 M q/N PUM FL 0 W FROM CENTRIFUGAL PUMP BACKGROUND OF THE PRESENT INVENTION The present invention relates to pumping units having a high head rise and more particularly to multiple-stage centrifugal pumps. High pressures have been obtainable in multiple-piston fluid pumps, but the pulsating output of such units has been found to be deleterious in many applications sometimes causing cracking and failure of the pump parts.

While it has been possible to obtain these high pressures from centrifugal pumping units, it frequently occurs that an unreasonable number of stages is required to effect the necessary head rise.

Attempts to achieve the high head rise in a few number of stages, e.g., in a two-stage centrifugal pump, have been found to result in excessive loading of the seals associated with the second-stage unit. That is, the high-pressure drop across the second-stage seal has required a close-clearance, high-differential pressure seal and even with such seals, head loss associated therewith has been found to significantly reduce the efficiency of the second stage.

It is therefore a primary object of the present invention to improve the sealing efficiency in a high-pressure centrifugal pump assembly.

SUMMARY OF THE PRESENT INVENTION In accordance with the present invention a two-stage centrifugal pump assembly is provided in which the pressure drop across mechanical seals in the second stage is significantly reduced by the provision of a plurality of sealing impellers in the second stage permitting higher pressure rise in the centrifugal pump than heretofore possible in a two stage unit.

The second-stage unit of the present invention includes a main centrifugal impeller with an axial inlet and a generally tangential outlet for the main flow received from the first stage. The first and second stage are arranged in back-to-back fashion driven by a common drive shaft through a gearbox. The portion of the shaft extending into the secondstage unit and driving the second-stage impeller carries two mechanical seals that do not require a particularly close clearance characteristic because of the provision of a plurality of centrifugal seal impellers mounted between the mechanical seals and the second-stage impeller on the second-stage shaft portion. These sealing impellers in conjunction with stationary housing vanes are arranged to serially direct sealing fluid through the sealing impellers toward the inboard side of the main secondstage impeller. The sealing impellers develop a sufficient pressure rise so that the exit seal flow from the last sealing impeller is at a pressure greater than the inlet pressure to the secondstage main impeller, which of course, is substantially the outlet head from the first stage. Since both the firstand second stage main impellers are open impellers, the pressure on both sides thereof, i.e., both the inlet side and the shaft side adjacent the sealing impellers is substantially equal, so that the sealing flow from the last stage of the sealing impellers prevents main flow leakage in an inboard direction from the main impeller. It is this arrangement that reduces the pressure drop across the mechanical seals which, as mentioned above, are located on the inboard side ofthe sealing impellers, i.e., on the side ofthe sealing impellers remote from the main impeller on the second stage.

Each of the impellers includes a disc rotatably fixed to the shaft with the plurality of radial vanes facing toward the mechanical seal, i.e., away from the main second-stage impeller, and a separate housing cavity having stationary radial blades on the main impeller side of the sealing impeller for directing fluid from the sealing impeller to the next succeeding sealing impeller.

In one embodiment of the present invention a sealing fluid circulation circuit is provided that prevents heating and boiling in the centrifugal seal impellers eliminating or ameliorating the conventional problems in dynamic seals of boiling, mineral deposits and instability. Toward this end a main passage is provided in the first-stage unit on the inboard side of the main first-stage impeller for bleeding a portion of the main pump fluid from the first stage to an inlet passage in the second stage which directs sealing fluid to the sealing impellers.

In another embodiment of the present invention sealing fluid is provided from an independent source of buffer fluid which enters the second-stage unit through a passage which directs this buffer fluid to the sealing impellers. Since the sealing impellers have a sufficient head rise to preclude main pump flow along the second-stage shaft, the main pump fluid is effectively isolated to contact only with the main impeller and thus corrosive fluids may be pumped with this arrangement.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a longitudinal section of the first stage of the present two-stage centrifugal pump assembly; and

FIG. 2 is a longitudinal section of the second stage of the present centrifugal pump assembly.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawings, including both FIGS. 1 and 2, the present centrifugal pump assembly is seen to include generally a first-stage unit 12 and a second-stage unit 14 driven by a common shaft 16 in a gearbox 18 disposed between the firstand second-stage units 12 and 14. Flow entering the first stage 12 at first-stage inlet 20 is compressed and delivered through an outlet and a crossover pipe (not shown) to inlet 22 associated with the second-stage unit 14. The second-stage unit further compresses the main pump fluid and directs it out second-stage outlet 24 shown in dotted lines in FIG. 2.

The first-stage unit 12 is provided with mechanical seals 26 and 28 and a single centrifugal seal impeller 30.

The second-stage unit 14 is provided with mechanical seals 32 and 34 and a plurality of centrifugal sealing impellers 36 which reduce the pressure drop across mechanical seal 34.

Referring to the construction of the first-stage unit 12 in more detail, the gearbox 18 has a side mounting plate 37 which receives a bearing 38 supporting high-speed shaft 16. Shaft 16 may, for example, rotate in the l0,00030,000 rpm. range. Affixed to the gearbox plate 37 is a housing member 40 having a central opening 42 that receives a seal support plate 43, a manifold plate 44 and an impeller backplate 45. The seal plate 43 is fixed to the housing 40 by fasteners 47 and the manifold plate 44 and backplate 45 are flxed within the housing opening 42 by elongated threaded fasteners 49.

A cover plate 51 surrounds first-stage impeller 52 and defines the free end shroud therefor, and also receives an inlet throat insert 53 having a conical portion 55.

The first-stage impeller 52 is provided with a conical hub inducer 56 of the type shown in the D. H. Silvern US. Pat. No. 3,299,821, filed Aug. 2l, 1964, entitled Pump inducer," assigned to the assignee of the present invention.

The impeller 52 is an open" impeller having free inboard and outboard blade edges and consists of a plurality of generally radially disposed blades 58 which taper outwardly, complementary to the conical shroud surface 60 defined by cover 51. The impeller blades 58 are retained in position on shaft 16 by a double-end-threaded fastener 62, threadedly engaged at one end in the inducer hub 63 and at the other end in shaft 16 locking lip portions 63 of the blades 58 therebetween.

The inboard mechanical seal 28 includes a relatively stationary seal member 65 slideably engaging a rotatable seal member 66. Mechanical seal 28 serves the function primarily ofsealing the pump unit 12 from the gearbox 18.

Mechanical seal 26 consists of a relativelystationary seal member 68 fastened to seal plate 43 by fasteners 70, which slidably engages a relatively rotatable seal. member 72 on shaft 16. The seal 26 serves the primary function of sealing the pump fluid or the seal fluid on the outboard side of the seal 26. As noted above, it is a primary object of the present invention to reduce the pressure drop across seal 26 and more particularly its counterpart in the second-stage pump unit 14 which will appear more clearly hereinbelow.

Seal plate 43 may be provided with a radial passage 74 for the purpose of delivering seal flush fluid between the seals 26 and 28.

In one embodiment of the present invention a portion of the main pump fluid from the first-stage unit 12 is conveyed to scaling impellers in the second-stage unit 14. Toward this end main pump fluid leaking along shaft sleeve 76, which is at the same pressure as fluid at inlet due to the open construction of impeller 52, bypasses the first-stage outlet and is bled from the first stage through a stationary vane member 78, radial passage 79 in the vane member, and radial passage 80 in manifold plate 44 which is connected to a bleed pipe leading to and connected with passage 82 associated with second stage 14. Thus in one embodiment of the present invention main pump fluid at inlet pressure is delivered from the firststage unit 12 to the sealing impellers associated with the second-stage unit 14 in a manner to reduce or eliminate heating and boiling in the second-stage seals.

The second-stage centrifugal pump unit 14 is generally similar in construction to the first-stage unit so that the common portions thereof will be described in somewhat less detail hereinafter.

The opposite sidewall 84 of the gearbox rotatably supports shaft 16 in bearing 86 and also carries a main housing 88 of the second-stage unit which has a central aperture 89 which receives a seal support plate 90, a manifold plate 91 and an impeller backplate 92. The sealing plate 90 is fixed to housing member 88 by threaded fasteners 94, and the manifold plate 91 and backplate 92 are fixed within central opening 89 by elongated threaded fasteners 94.

A cover plate 96 extends over the open end of housing member 88 and defines a shroud cavity 99 for second-stage centrifugal impeller 100. Similar to impeller 52, impeller 100 is an open impeller consisting of a plurality of blades 102 fixed on shaft 16 by a threaded fastener 104. Cover member 96 has a conical inlet insert 106 seated centrally therein which communicates with an inlet opening 108 in inlet plate 110 fastened to the cover 96.

For the purpose of flushing the area or chamber 112 between the seals 32 and 34, a radial passage 114 is provided in sea] support plate 90 which, in turn, communicates with a normally plugged radial passage 116 in housing 88.

Seal 32 serves the primary purpose of sealing the pump from the interior of the gearbox and consists of a stationary seal member 120 which slidably engages a relatively rotatable seal member 121 on shaft 16.

Seal 34 serves the primary function of sealing the pump fluid associated with the second stage or the seal fluid associated with seal impellers 36 and includes a relatively stationary seal member 122 fixed to seal plate 90 by fasteners 123. Seal member 122 slidably engages a rotatable seal member 125 on shaft 16. It is a primary object of the present invention to improve the efficiency of the present two-stage pump by reducing the pressure drop across the second-stage mechanical seal 34.

Toward this end the impeller set 36 is provided including a first impeller 128, a second impeller 129 and a third impeller 130. More than three impellers may be provided, if desired, in set 36. The impellers in set 36 are identical in construction and are seen to include a hub portion 132, a radially extending shroud 133 and a plurality of radially extending vanes 136 which face inboard with respect to the gearbox 18, or in a direction opposite the second-stage impeller 100. For the purpose of directing sealing fluid from one seal impeller to another, there is provided a plurality of stationary radially directed vanes 138 adjacent the outboard sides of the shroud plates 133 having a substantial radial clearance as at 140 to permit seal fluid flow to the radial innermost portion of the blades associated with the next impellers 129, 130. The impeller set 36 is constructed to achieve a sufficient head rise so that seal fluid flowing from the last impeller is at a pressure in excess of the main pump inlet flow in second-stage inlet 108 or the exit flow from the first-stage pump unit 12. This prevents the main pump fluid on the inboard side of blades 102 from egressing inwardly along nosepieces 141.

As described above, main pump flow from the first stage is bypassed to passage 82, leading to the inlet of the sealing impeller 128 through passages 142 in manifold plate 91 which connects with radial passage 143 in the same plate, radial passages 145 in a stationary nozzle plate 147 and through the radial clearance between nozzle plate 147 and the hub 132 associated with impeller 128.

Thus in the embodiment of the invention. thus far described, main pump fluid is bypassed from the first stage to the second-stage centrifugal sealing impellers 36 and this seal fluid circulation arrangement prevents heating and boiling in the dynamic seal impellers so that the usual centrifugal dynamic seal problems of boiling, mineral deposit and instability are ameliorated or eliminated entirely.

ln another embodiment of the present invention a somewhat modified scheme is provided for circulating seal fluid in both the first and second stages. This scheme is particularly suitable for use in centrifugal pumps which pump a corrosive fluid. To reduce the problem of corrosion of the pumping elements a compatible buffer seal fluid is employed. Toward this end the bleed passage 80 associated with the firststage unit 12 is blocked by a suitable plug and a compatible buffer fluid is supplied through another passage 151 in manifold member 44 to a space 153 between the mechanical seal and a seal impeller 156. Seal impeller 156 is somewhat larger than the seal impellers in set 36 and serves to pump fluid from chamber 153 through the stationary nozzle member 78, which has radial vanes thereon, outwardly along the sleeve 76. The impeller 156 is constructed to deliver fluid along sleeve 76 at a somewhat higher head than the inlet flow at the exit of inducer 56 so that in this manner the entire shaft assembly, excluding the first-stage impeller 52 is isolated from the fluid being pumped by the main impeller. Thus if a corrosive fluid is being pumped, the entire inboard section of the first-stage pump to the right of the impeller shown in FIG. 1 will be isolated from this corrosive fluid.

A similar arrangement is provided in this embodiment for the second-stage pump 14. More particularly, the main pump flow bypass passage 82 is plugged and a compatible buffer fluid is supplied to passage 160 in manifold plate 91 which communicates with an annular chamber 161 between the mechanical seal 34 and an additional centrifugal seal impeller 163. Impeller 163 is somewhat larger than the impellers 128, 129 and 130 but is similar in construction thereto in that it includes radial blades facing inboard and cooperates with stationary vane member 47 having radial vanes 166 adjacent the outboard side of the impeller 163.

The impeller 163 draws the compatible buffer fluid from chamber 161 and directs it through the stationary blades 166 and hence serially through the impellers 128, 129 and 130. In this manner the main, second-stage impeller 100 is isolated from the inboard portions of the second stage and a corrosive fluid may readily be pumped by the assembly. Moreover as described above, the impeller set 36, in this embodiment, serves the same function of reducing the pressure drop across the mechanical seal 34 sufflciently so that the second stage 14 can develop higher pressures than heretofore possible.

We claim:

1. A centrifugal pump assembly, comprising:

a first-stage pump unit having a main impeller, an inlet and an outlet, a second-stage pump unit having a main impeller, an inlet and outlet, said firstand second-stage pump units having common drive means, said secondstage pump unit having inboard seal means, centrifugal seal means between said inboard seal means and said second-stage impeller, centrifugal seal means being constructed to deliver fluid toward said second-stage impeller at a pressure in excess of the first-stage discharge pressure and means responsive to flow from the first-stage pump unit for delivering fluid to the centrifugal seal means of the second-stage pump unit.

2, A centrifugal pump assembly as defined in claim 1, wherein said centrifugal seal means includes a plurality of impellers coaxially positioned with respect to the second-stage impeller.

3. A centrifugal pump assembly as defined in claim 2, wherein each of said seal impellers includes an annular backplate with radial blades facing said inboard seal means.

4. A centrifugal pump assembly, comprising: a first-stage pump unit having an impeller, and axial inlet on one side of the impeller and an outlet, a second-stage pump unit having an impeller and an axial inlet connected to the first-stage outlet on one side of the impeller, and an outlet, said second-stage impeller being supported and driven by a shaft, inboard seal means sealing said shaft on the side of said second-stage impeller opposite the second-stage inlet, centrifugal seal means on said shaft between said inboard seal means and the secondstage impeller, and means for conveying a portion of the main pump fluid from the inlet of the first-stage pump unit to the centrifugal seal means to minimize boiling of the fluid as it passes through the centrifugal seal means.

5. A centrifugal pump assembly, comprising: a first-stage pump unit having an impeller, an axial inlet on one side of the impeller and an outlet, a second-stage pump unit having an impeller and axial inlet connected to the first stage outlet on one side of the impeller, and an outlet, said first and second pump units having common drive means, said second-stage impeller being supported and driven by a shaft, inboard seal means supporting said shaft on the side of said second-stage impeller opposite the second-stage inlet, centrifugal seal means on said shaft between said inboard seal means and the second stage impeller, and means responsive to inlet flow to the first-stage pump unit for conveying a cool buffer fluid to said centrifugal seal means to minimize boiling.

6. A centrifugal pump assembly as defined in claim 4, wherein said centrifugal seal means includes a plurality ofimpellers each coaxially mounted on said shaft and having a plurality of radial blades facing said inboard seal means, said impellers being arranged to deliver fluid serially from one seal impeller to another.

7. A centrifugal pump assembly as defined in claim 6, wherein said seal means includes a plurality of radial stationary blades adjacent the main impeller side of each of the centrifugal seal means impellers.

8. A centrifugal pump assembly as defined in claim 4, ineluding a shaft supporting and drivingly connected to said first-stage impeller, said first'stage impeller being an open im peller so that the pressure along the shaft is substantially the same as the first-stage inlet pressure, said means for conveying inlet fluid including a bypass passage communicating with the first-stage shaft for bleeding pump fluid to the second-stage centrifugal seal means.

9. A centrifugal pump assembly as defined in claim 5, including passage means in said second-stage unit between said inboard seal means and the centrifugal seal means for conveying the buffer fluid.

10. A centrifugal pump assembly as defined in claim 9, said first-stage unit having an impeller shaft and an inboard bearing surrounding said shaft, centrifugal seal means on said firststage shaft between said first-stage inboard seal and said firststage impeller, and passage means connected with said firststage unit for directing buffer seal fluid to said first-stage centrifugal seal means.

11. A centrifugal pump assembly, comprising: a first-stage centrifugal pump having housing means, a main impeller mounted in said housing means, a shaft supported for rotation in said housing means, an axial inlet to the impeller in the housing, an outlet for the impeller in said housing means, a

mechanical seal surrounding said shaft in said housing means, bleed passage means in said housing means for conveying pumped fluid from between the impeller and the mechanical seal, a second-stage centrifugal pump having a housing means, a main impeller mounted in said housing means, a shaft in said housing means rotatably driving said impeller, an axial inlet in said housing means for the second-stage impeller, an outlet in said housing means for said secondstage impeller, a mechanical seal on the inboard end of the second-stage shaft, a plurality ofseal impellers on said shaft between said second-stage impeller and said second-stage mechanical seal, said seal impellers having a plurality of radial vanes facing said second-stage mechanical seal, said housing means being constructed to deliver seal fluid radially from one seal impeller to another, and passage means between said second-stage mechanical seal and said seal impellers for delivering pump fluid from said first-stage pump means to the second-stage seal impellers.

12. A centrifugal pump assembly, comprising: a first-stage centrifugal ump unit having a housing means, an impeller rotatably mounted in said housing means, a shaft supporting said impeller in said housing means, an axial inlet in said housing means for said impeller, an outlet in said housing means, a mechanical shaft seal surrounding said shaft at the inboard end thereof, a seal impeller on said shaft between said mechanical seal and said impeller, passage means in said housing for conveying a relatively cool buffer fluid between said mechanical seal and said seal impeller, a second-stage centrifugal pump unit having a housing means, common means for driving said first and second-stage pump units, a seal impeller rotatably mounted in said second-stage housing means, and means responsive to rotation of said first-stage centrifugal pump unit for delivering sealing fluid to the second-stage seal impeller. 

1. A centrifugal pump assembly, comprising: a first-stage pump unit having a main impeller, an inlet and an outlet, a second-stage pump unit having a main impeller, an inlet and outlet, said first- and second-stage pump units having common drive means, said second-stage pump unit having inboard seal means, centrifugal seal means between said inboard seal means and said second-stage impeller, centrifugal seal means being constructed to deliver fluid toward said secondstage impeller at a pressure in excess of the first-stage discharge pressure and means responsive to flow from the firststage pump unit for delivering fluid to the centrifugal seal means of the second-stage pump unit.
 2. A centrifugal pump assembly as defined in claim 1, wherein said centrifugal seal means includes a plurality of impellers coaxially positioned with respect to the second-stage impeller.
 3. A centrifugal pump assembly as defined in claim 2, wherein each of said seal impellers includes an annular backplate with radial blades facing said inboard seal means.
 4. A centrifugal pump assembly, comprising: a first-stage pump unit having an impeller, and axial inlet on one side of the impeller and an outlet, a second-stage pump unit having an impeller and an axial inlet connected to the first-stage outlet on one side of the impeller, and an outlet, said second-stage impeller being supported and driven by a shaft, inboard seal means sealing said shaft on the side of said second-stage impeller opposite the second-stage inlet, centrifugal seal means on said shaft between said inboard seal means and the second-stage impeller, and means for conveying a portion of the main pump fluid from the inlet of the first-stage pump unit to the centrifugal seal means to minimize boiling of the fluid as it passes through the centrifugal seal means.
 5. A centrifugal pump assembly, comprising: a first-stage pump unit having an impeller, an axial inlet on one side of the impeller and an outlet, a second-stage pump unit having an impeller and axial inlet connected to the first stage outlet on one side of the impeller, and an outlet, said first and second pump units having common drive means, said second-stage impeller being supported and driven by a shaft, inboard seal means supporting said shaft on the side of said second-stage impeller oPposite the second-stage inlet, centrifugal seal means on said shaft between said inboard seal means and the second stage impeller, and means responsive to inlet flow to the first-stage pump unit for conveying a cool buffer fluid to said centrifugal seal means to minimize boiling.
 6. A centrifugal pump assembly as defined in claim 4, wherein said centrifugal seal means includes a plurality of impellers each coaxially mounted on said shaft and having a plurality of radial blades facing said inboard seal means, said impellers being arranged to deliver fluid serially from one seal impeller to another.
 7. A centrifugal pump assembly as defined in claim 6, wherein said seal means includes a plurality of radial stationary blades adjacent the main impeller side of each of the centrifugal seal means impellers.
 8. A centrifugal pump assembly as defined in claim 4, including a shaft supporting and drivingly connected to said first-stage impeller, said first-stage impeller being an open impeller so that the pressure along the shaft is substantially the same as the first-stage inlet pressure, said means for conveying inlet fluid including a bypass passage communicating with the first-stage shaft for bleeding pump fluid to the second-stage centrifugal seal means.
 9. A centrifugal pump assembly as defined in claim 5, including passage means in said second-stage unit between said inboard seal means and the centrifugal seal means for conveying the buffer fluid.
 10. A centrifugal pump assembly as defined in claim 9, said first-stage unit having an impeller shaft and an inboard bearing surrounding said shaft, centrifugal seal means on said first-stage shaft between said first-stage inboard seal and said first-stage impeller, and passage means connected with said first-stage unit for directing buffer seal fluid to said first-stage centrifugal seal means.
 11. A centrifugal pump assembly, comprising: a first-stage centrifugal pump having housing means, a main impeller mounted in said housing means, a shaft supported for rotation in said housing means, an axial inlet to the impeller in the housing, an outlet for the impeller in said housing means, a mechanical seal surrounding said shaft in said housing means, bleed passage means in said housing means for conveying pumped fluid from between the impeller and the mechanical seal, a second-stage centrifugal pump having a housing means, a main impeller mounted in said housing means, a shaft in said housing means rotatably driving said impeller, an axial inlet in said housing means for the second-stage impeller, an outlet in said housing means for said second-stage impeller, a mechanical seal on the inboard end of the second-stage shaft, a plurality of seal impellers on said shaft between said second-stage impeller and said second-stage mechanical seal, said seal impellers having a plurality of radial vanes facing said second-stage mechanical seal, said housing means being constructed to deliver seal fluid radially from one seal impeller to another, and passage means between said second-stage mechanical seal and said seal impellers for delivering pump fluid from said first-stage pump means to the second-stage seal impellers.
 12. A centrifugal pump assembly, comprising: a first-stage centrifugal pump unit having a housing means, an impeller rotatably mounted in said housing means, a shaft supporting said impeller in said housing means, an axial inlet in said housing means for said impeller, an outlet in said housing means, a mechanical shaft seal surrounding said shaft at the inboard end thereof, a seal impeller on said shaft between said mechanical seal and said impeller, passage means in said housing for conveying a relatively cool buffer fluid between said mechanical seal and said seal impeller, a second-stage centrifugal pump unit having a housing means, common means for driving said first- and second-stage pump units, a seal impeller rotatably mounted in said second-stage housing means, and means responsivE to rotation of said first-stage centrifugal pump unit for delivering sealing fluid to the second-stage seal impeller. 